An internal combustion engine for a vehicle is provided with a variable valve control apparatus which adjusts valve characteristic (valve timing, valve lift amount, working angle and the like) of an intake valve and an exhaust valve in order to improve its output and fuel economy and to reduce emission. In a system provided with a variable valve lift control apparatus driven by an electric motor, the control duty (current value) of the electric motor is controlled by PI control or PID control in such a manner that an actual lift amount agrees with a target lift amount.
JP-10-220619A shows an EGR control system in which a motor-driven EGR valve is feedback controlled by PID control in order to adjust an EGR amount. When a difference between a target opening/closing position and an actual opening/closing position of the EGR valve is within a permissible range, the I-gain (integral gain) is cleared to avoid an integration of a steady-state deviation due to the I-action (integral control action) so that an opening/closing vibration of the EGR valve is restricted.
As shown in FIG. 2, in the variable valve lift control apparatus of the motor drive type, a hysteresis characteristic may arise in the relationship between a control duty and a lift amount according to reaction forces which are received from the cam driving the intake valve. Due to this hysteresis characteristic, the control duty with respect to the same lift amount is varied between a case that the lift amount is increasing and a case that the lift amount is decreasing. Even when the variable valve lift control apparatus is in a steady condition, that is, even when the lift amount is maintained at a constant value, the control duty is a large value or a small value according to a prior control direction of the lift amount. When the control duty is the large value, more energy is consumed than a case where the control duty is a small value.
Generally, when calculating control duty by the PI control or the PID control, the difference of the control duty due to the hysteresis characteristic is adjusted mainly by the I-action (integral control action) which generates an output proportional to an integrated value of the difference between the target lift amount and the actual lift amount. Therefore, an increment of I-term (integral term) is equivalent to an increment of the control duty due to the hysteresis characteristic.
Even if the increment in the I-term is restricted by clearing the I-gain to avoid the integration of a difference by the I-action in a situation where the difference between the target lift amount and the actual lift amount is within the permissible range, only a further increment in I-term is restricted after the valve lift amount is maintained at the constant value. The increment in I-term until the valve lift amount becomes constant still remains. Hence, when the control duty is a large value due to the hysteresis characteristic, the control duty of large value is continued, which increases the energy consumption.